Alkylaromatic deprotonation

The increase in the electron acceptor concentration in a solution transforms the reaction of fluorene oxidation into regime when the first stage (deprotonation of fluorene) limits the chain process. For the oxidation of other alkylaromatic hydrocarbons, see Refs. [292 304]. 

At pH <4 both 2"+ and 3,+ undergo C-H deprotonation as the exclusive reaction while in basic solution they behave as oxygen acids undergoing OH-induced OH deprotonation in a diffusion controlled process. Results indicate that in alkylaromatic radical cations, overlap between the scissile bond and the jt-system containing the unpaired electron is a fundamental requirement for the bond cleavage. The observation that for both 2 and 3, the 1,2-H atom shift occurs more rapidly than C-C ft-scission whereas the radical derived from OH de-... 

The transition state for the deprotonation of an alkylaromatic radical cation might be described in terms of the mesomeric structures in Scheme 31. In structure I the positive charge is delocalized on the aromatic ring whereas structures II and III represent the homolytic and heterolytic C-H bond cleavage modes, respectively. 

The intramolecular selectivity in the deprotonation of alkylaromatic radical cations has been investigated in a series of 5-X-l,2,3-trimethylbenzene radical cations the results showed that spin and/or charge density at the scissible C H bond can strongly influence the kinetic acidity, which is, accordingly, very sensitive to the nature and position meta or para) of ring substituents (Scheme 32) [149]. 

As remarked previously, the deprotonation rate of an alkylaromatic radical cation can be influenced by the stereoelectronic effect (Scheme 27). In this respect, Tolbert provided convincing evidence for the operation of stereoelectronic effects in the deprotonation of 9-alkylanthracene and 9,10-dialkylanthracene radical cations... 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

As previously discussed, removal of an electron from the 7r-system of alkylaromatic substrates leads to a dramatic increase in the acid strength of the C -H bonds, as clearly shown by the pA a between -11 and —13 estimated for the deprotonation of toluene radical cation in acetonitrile [130]. The presence of a positive charge on the aromatic ring can also influence, however to a much smaller extent, the acidity of groups which are further spaced from the aromatic ring such as OH and CO2H as... 

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